The fuel efficiency of a motor vehicle can be considerably enhanced with a hybrid system including an electric machine coupled to the engine, a high voltage battery pack, and a power electronics system for interconnecting the electric machine, the battery pack and the electrical loads of the vehicle. The electric machine is operable in a generating mode to charge the battery pack and supply power to various electrical loads, and in a motoring mode to crank the engine and to augment the engine power output. Various drive arrangements can be used to propel the vehicle. For example, the engine can be coupled to the drive wheels through a conventional drivetrain, and/or one or more electric propulsion motors can be used.
FIG. 1 illustrates an example of a hybrid vehicle system including an engine 10 that is mechanically coupled to a set of drive wheels 12 through a transmission (T) 14 and differential gearset (DG) 16. The hybrid vehicle system includes an AC electric machine 18, a main 120-volt battery pack 20, and a power conversion system 22. The electric machine 18 is selectively operable in generating and motoring modes, and is mechanically coupled to the engine 10, either directly or by way of a drive belt. The power conversion system 22 includes a high voltage DC bus 24, one or more bus capacitors 26 for maintaining the bus voltage, a power coupler 28 coupling the positive side of high voltage bus 24 to the battery pack 20, an inverter 30 coupling high voltage bus 24 to the electric machine 18, a DC-to-DC converter 32 coupling high voltage bus 24 to a low voltage DC bus 34, and a Power Control Unit (PCU) 36 for controlling the operation of inverter 30 and DC-to-DC converter 32. The power coupler 28 may be implemented with a fuse, with a controlled element such as a relay, or with a fuse in series with a relay; and in mechanizations where the power coupler 28 includes a relay, its on/off state is controlled by PCU 36 as indicated in FIG. 1. The low voltage DC bus 34 is used to supply power to various 12-volt electrical loads 38 of the vehicle, and an auxiliary 12-volt storage battery 40 is coupled to the low voltage bus 34 for maintaining the bus voltage and temporarily supplying power to the loads 38 in the event of a system failure.
In mechanizations where the power coupler 28 includes a relay or other controlled element, the PCU 36 is programmed to open the relay when a failure mode requiring battery pack disconnection is detected. In order to minimize the current that the relay must break and to prevent load-dump transient voltages, the PCU 36 ordinarily powers down the inverter 30 and DC-to-DC converter 32 prior to opening the relay. However, once the relay is open and the battery pack 20 is off-line, there is insufficient reserve electrical power in the bus capacitor 26 to re-activate the electric machine 18, and the only source of power for the electrical loads 38 is the auxiliary storage battery 40. A similar situation occurs in mechanizations where the power coupler 28 includes a fuse that opens in response to an over-current condition, except that the inverter current automatically collapses when the fuse opens. In either case, the battery pack disconnection significantly limits the post-failure range of the vehicle because certain electrical loads such as the engine ignition system are required for continued operation of engine 10, and the auxiliary battery 40 can only power such loads for a limited period of time. Accordingly, what is needed is a way of utilizing the generating capability of the electric machine 18 to power the vehicle following a controlled or uncontrolled disconnection of battery pack 20 from the high voltage bus 24.